The invention concerns a frequency converter for an electromotor with an intermediary circuit, in which a braking circuit with a switch and a load is arranged.
Frequency converters normally consist of a rectifier rectifying a one- or multi-phase AC-voltage and supplying it to an intermediary circuit. Alternatively, the intermediary circuit can also be supplied from a different DC source. The intermediary circuit supplies an inverter, which again supplies the connected motor with a voltage with the desired frequency. When an interruption of the motor operation is desired, the voltage/frequency relation is reduced, so that the current changes its direction. Now the frequency converter no longer supplies current. In this case the motor acts as a generator supplying power back to the frequency converter, as the terminal voltage in the motor is higher than that in the inverter. For the braking of the motor the electrical energy fed back is often led through a load, by which the supply of electrical energy to the load is controlled by a switch, also called a braking chopper. Loads can be defined as all sorts of devices converting electrical energy into different energy forms, such as heat.
Such a frequency converter is known from for instance U.S. Pat. No. 5,420,491. Here, the load is an ohmic resistor.
For motors with high inertia torques the amount of energy fed back to the frequency converter is often relatively high. Then the load can no longer in all cases be arranged inside the housing of the frequency converter, meaning that it has to be arranged outside the housing. In this case, for instance, the cooling is better. Also, other sorts of energy conversion of electrical energy into other types of energy are easier to realise. However, arranging the load outside the frequency converter bears the disadvantage that the load is exposed to the environmental conditions. Thus, humidity and mechanical or electromechanical conditions may influence the load, which may lead to electrical failures, such as earth faults or short-circuits.
For this reason, several countries prescribe that such electronic devices must be provided with a protective device, as for instance a fault-current protective switch (earth leakage breaker=FI-switch).
Such FI-switches are different, depending on the demands they have to meet. Thus, the IEC 755, chapter 4.2.10, for instance, distinguishes between type A switches and type B switches. The type B switches can, for example, detect a direct current component in the earth leakage current, which is the case when for instance an earth fault occurs in the load, which is connected to a DC intermediary circuit. This results in a current flow comprising a direct current component. Such direct current components cannot be detected by type A FI-switches.
However, a type B FI-switch is relatively expensive, and bears the disadvantage that the installation is complicated and expensive too. Besides, the connection of other FI-switches before type B FI-switches is not permitted.
Thus, it is the purpose of the invention to reduce the demands on protective measures in connection with a frequency converter.